Isoproterenol, a B-adrenergic agonist, has been known for years to be effective in decreasing aqueous humor (AH) production in man. Most recently, the B-adrenergic antagonist, timolol, has been found to have effects identical to the agonist. Our studies on intact, anesthetized cats and on enucleated arterially perfused eyes indicate that the action of timolol may not be mediated by B-adrenergic blockade. The precise mechanism of its action is still being pursued. The actions of the prostaglandins (PG) E1, E2 and F2 alpha as well as of arachidonic acid (AA) were studied in the enucleated, arterially perfused eye. Each of the PGs were found to decrease AH production while AA caused the formation rate of AH to become elevated. A vascular mechanism for each of these responses appears very probable.